Photosensitive elements for use in electrophotography and method of manufacturing same

ABSTRACT

A PHOTOSENSITIVE ELEMENT OF THE MULTI-LAYER TYPE IS PREPARED BY FIRST DEPOSITING A LAYER OF A SE-TE ALLOY CONTAINING MORE THAN 20% OF TE TO FORM A FIRST LAYER, CODEPOSITING THE SE-TE ALLOY AND SE TO FORM A SECOND LAYER AND FINALLY DEPOSITING SE ON THE SECOND LAYER TO FORM A THIRD LAYER.

United States Patent O US. C]. 96-15 8 Claims ABSTRACT OF THE DISCLOSURE A photosensitive element of the multi-layer type is prepared by first depositing a layer of a Se-Te alloy containing more than 20% of Te to form a first layer, codepositing the Se-Te alloy and Se to form a second layer and finally depositing Se on the second layer to form a third layer.

BACKGROUND OF THE INVENTION This invention relates to a method of manufacturing photosensitive elements for use in electrophotography and more particularly to a method of manufacturing photosensitive elements of multi-layered construction by a vapour deposition technique.

In recent years a technique of utilizing the charge trapping effect in photosensitive elements for electrophotography has been introduced with remarkable advantages. A method of electrophotography has been provided utilizing a photosensitive element comprising a layer of photoconductive material and a thin layer of transparent and highly insulative material integrally bonded to one surface of the photoconductive layer to block the migration of charge carriers thus creating the persistent internal polarization effect. The advantages of this method in particular can be enhanced by providing a special photoconductive layer for trapping electric charge near the interface between the highly insulative or current blocking layer and the photosensitive layer.

To provide such a special layer for trapping electric charge, there were proposed a number of methods. According to one method a pigment containing layer was applied, and according to another method a photoconductive layer having a plurality of trap levels was formed by vapour deposition technique. This invention relates to an improvement of the method of manufacturing photosensitive elements by the vapour deposition technique.

The vapour deposition technique is particularly suitable for forming a special photoconductive layer having characteristics as described above on the conventional photoconductive layer because this technique assures very smooth surface and uniform thickness of the deposited thin film or layer. However, in most cases mere deposition of the photoconductive layer containing a number of trap levels upon a vapour deposited conventional photoconductive layer does not result in the desired characteristics. For example, where a photosensitive element was fabricated by a method comprising the steps of vapour depositing upon a metal substrate Se-Te alloy containing more than 20% of Te to a thickness of about 25 microns, vapour depositing upon the Se-Te layer a thin layer of Se or Se rich material to a thickness of about 1 micron and applying on the surface of the Se layer a layer of insulating material, for example polycarbonate to a thick-' ness of about 10 microns, the following results were noted. More particularly, in order to form an electrostatic latent image, the surface of the highly insulative layer 3,788,291 Patented Jan. 2, 1973 of polycarbonate is charged by means of corona discharge until the surface potential thereof is about'l000 volts. During or immediately after this charging step the surface of the photosensitive element is subjected to uniform light to create a polarization charge of uniform intensity in the interior of the photoconductive layer. This charge is expected to be injected into the Se trap layer from the Se-Te photoconductive layer and trapped in the trap levels. The surface of the highly insulative layer is then charged by means of corona discharge of positive polarity while, at the same time, a light image is projected upon the photoconductive layer. During this second step, at portions of the photosensitive element corresponding to bright portions of the light image the polarization charge that has been trapped by trap levels is reexcited by light stimulation whereby to migrate toward a backing electrode secured to the rear side of the photoconductive layer under the influence of the field applied in the second step. At portions of the photosensitive element corresponding to dark portions of the light image, however, the trapped charge does not migrate but remains in the trapped state so that migration of the charge in the photoconductive layer is very small. Consequently, at portions corresponding to bright portions of the light image the surface of the photosensitive element is charged with a large quantity of positive charge but at portions corresponding to dark portions of the light image the quantity of charge is very small. Thus, an electrostatic latent image corresponding to the light image is formed on the surface of the highly insulative layer. A recently developed method of forming an electrostatic latent image on a photosensitive element comprising a highly insulative layer and a photoconductive layer utilizes the process steps outlined above. In that method the special photoconductive layer containing a number of trap levels was provided leading to the expectation of the occurrence of the above described phenomena. Actually, however, the above described photoconductive layer did not provide the expected results but rather decreased the photosensitivity, thus not contributing any improvement in the intensity of the latent image. These undersirable results can be attributed to the following reasons. First, it is considered that there is formed a barrier of considerably high potential at the interface between the Se-Te layer and the Se layer. This potential barrier functions to prevent injection of charge carriers from the Se-Te layer into the Sc layer. In such a case the desired trapping of the charge in the Se layer cannot be obtained and most of the polarization occurs in the Se-Te layer of low dark resistance. Second, during the second step in which positive field is applied across the photosensitive element, the dark resistance of the Se-Te layer is low so that the polarization of a polarity corresponding to that of the applied second field is formed at a high speed. On the other hand, the Se layer operates with a quite different time constant, and moreover, since nearly all of the projected light image is absorbed by the Se layer these two layers operate quite independently. For this reason, provision of the Se layer does not result in any novel effect.

SUMMARY OF THE INVENTION It is therefore an obect of this invention to provide a novel method of manufacturing a photosensitive element having a charge trap layer and a photoconductive layer by vapour deposition technique.

Another object of this invention is to provide a photosensitive element comprising a first photoconductive layer and a second photoconductive layer characterized in that there is no potential barrier at the interface between these two layers which prevents injection of charge carriers.

According to this invention a photosensitive element of multilayer construction for use in electrophotography is prepared by a method comprising the steps of vapour depositing a Se-Te alloy containing more than 20% of Te to form a first layer, concurrently vapour depositing upon the first layer Se and said Se-Te alloy to form a second layer, and vapour depositing upon the second layer Se to form a third layer. Dependent upon the type of the method and apparatus for forming electrostatic latent images on the photosensitive element, a highly insulative layer may be provided on the surface of the third layer and the first layer may be deposited upon an electroconductive substrate acting as a backing electrode Since the Se and Se-Te alloy are co-deposited to form an inter-diffused structure in the second layer, the undesirable potential barrier is not present between the first and thirdlayers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following specific example is given by way of illustration, and is not to be construed as limiting in any way the scope and spirit of the invention.

In a vapour deposition tank, two boats were disposed containing sources of vapour deposition. These boats were arranged such that their heating conditions were adjustable independently. Each of the boats were provided with a shutter to control the degree of vapour deposition as desired. A powder of a Se-Te alloy containing more than 20% of Te was put in one boat while a powder of Se was put in the other. After evacuating the tank to a vacuum of about torrs the boat containing the Se-Te alloy was heated quickly and when the alloy was melted completely, the shutter above the boat was opened to deposit the vapour of the Se-Te alloy upon an electrically conductive substrate heated to about 60 C. When the thickness of the deposited layer of the Se-Te alloy reached about 25 microns, the boat containing Se was heated rapidly and the shutter associated therewith was opened to effect concurrent deposition of the Se-Te alloy and Se. After about 30 seconds, the shutter associated with the boat containing the Sc-Te alloy was gradually closed for an interval of about 30 seconds. Thereafter, only Se was permitted to deposit for about 10 seconds. At the end of this deposition time, all shutters were closed to terminate deposition.

On the surface of the Sc layer, a layer of polycarbonate was applied to a thickness of about 10 microns to obtain a photosensitive element.

With this photosensitive element, a latent image was formed on the surface of the polycarbonate layer by a method comprising the steps of charging the photosensitive element to a potential of about 800 volts, applying a positive corona discharge for about 0.1 second and projecting a light image through the corona discharge electrode concurrently with the application of the positive corona discharge, said light image having a brightness of about three luxes at its bright portions. When measured under room light, the latent image showed a potential of 250 volts at portions corresponding to the bright portions of the light image and +300 volts at portions corresponding to dark portions thereof. When developed with a conventional charged developer powder, the latent image gave an intense visible image.

As can be noted from the foregoing example, after the first photoconductive, layer has been formed by vapour depositi0n,,the second or special photoconductive layer was formed without taking out into air the first photoconductive layer. Further, as the first and second photoconductive materials were deposited simultaneously for a certain interval to form a mutually diffused layer as the second layer, the formation of a potential barrier was prevented permitting easy exchange of charge carriers between the first and third layers, thus solving the problem described above.

While the above example was described in terms of the combination of a Se-Te alloy and Se, it should be understood that this invention is by no means limited to this particular combination but the invention can be equally applied to the preparation of multi-layered construction comprising any desired combinations of other materials, for example, combinations of CdS and ZnS, CdSe and Se and the like.

The charge trap layer can be formed on the rear side of a photoconductive layer instead of on the surface thereof. Further, as is well known in the art, a backing electrode may be formed on the rear side of the first photoconductive layer.

Further, this invention is also useful to prepare photosensitive elements for use in the conventional xerography, that is elements without highly insulative layers.

What is claimed is:

1. A photosensitive element of multi-layer construction for use in electrophotography comprising a conductive substrate, a first layer of Se-Te alloy containing more than 20% of Te on said substrate, a second layer of Se and Se- Te alloy forming an inter-diffused structure, and a third layer of Se.

2. A photosensitive element according to claim 8 including a highly insulative layer integrally formed on said third layer.

3. A photosensitive element according to claim 8 wherein said conductive substrate acts as a backing electrode.

4. A photosensitive element according to claim 1 wherein the content of said Se-Te alloy in said second layer decreases gradually from said first layer toward said third layer.

5. In a method of manufacturing a photosensitive element of multi-layer construction for use in electrophotography by means of vapor deposition technique, the improvement which comprises the steps of vapor depositing upon a conductive substrate a Se-Te alloy containing more than 20% of Te to form a first layer, concurrently vapor depositing upon said first layer Se and said Se-Te alloy to form a second layer, and vapor depositing upon said second layer Se to form a third layer.

6. The method of manufacturing a photosensitive element according to claim 5 wherein a highly insulative layer is integrally formed on said third layer.

7. The method of manufacturing a photosensitive element according to claim 5 wherein said substrate acts as a backing electrode.

8. The method of manufacturing a photosensitive element according to claim 7 wherein the content of said Se-Te alloy in said second layer decreases gradually from said first layer toward said third layer.

References Cited UNITED STATES PATENTS 3,041,166 6/1962 Bardeen 117-107 UX 3,102,828 9/1963 Courvoisier 117-107 X 2,687,484 8/1954 Weimer 96--1.5 X 2,962,376 11/1960 Schalfert 96-l.5 3,355,289 11/1967 Hall et al. 96-1.5 X

ALFRED L. LEAVITT, Primary Examiner C. K. WEIFFENBACH, Assistant Examiner US. Cl. X.R. l17l06 R, 215

.. 2 7? UNITED S'JYATES l YIENT OFFiCE s as),

I' W "\f r"\ 7 f; f' T \T' W CLLR L MEL/l i 11 Gr CQiuili L 5. {J11 Patent No. 3, 708, 291 Dated January 2 1973 lnvmtofls) Koichi Kinoshita, Takao Kawamura It is certified that error appears in the above--identified patent and that said Letters Patent are hereby corrected as shown below:

Co1umn-2,- line 41,- "undersira ble should be '-'undesira ble line 63, "obect" should be obje :t--

Claim 2, line 1, "claim 8" should be -claim l--- Claim 3 line 1, "claim 8" should be claim 1-- Signed and sealed this 20th day of November 1973';

(SEAL) Attest:

EDWARD M.FLETCHER,JR.- RENE .D. TEGTMEYER Attesting Officer Acting Commissioner of Patents 

